With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using a magnetic recording technology is continuously increasing at an annual rate of approximately 100%. In recent years, an information recording capacity exceeding 200 gigabytes per perpendicular magnetic recording medium with a 2.5-inch diameter for use in an HDD or the like has been demanded. To fulfill such a demand, an information recording density exceeding 400 gigabits per square inch is desired to be achieved.
To achieve a high recording density in a magnetic recording medium for use in an HDD or the like, a perpendicular magnetic recording type has been proposed in recent years. In a perpendicular magnetic recording medium for use in the perpendicular magnetic recording type, the axis of easy magnetization of a magnetic recording layer is adjusted so as to be oriented in a direction perpendicular to the base surface. In the perpendicular magnetic recording type, compared with a conventional in-plane recording type, it is possible to more suppress a so-called thermal fluctuation phenomenon, in which thermal stability of a recording signal is impaired because of a superparamagnetic phenomenon to cause the recording signal to be lost, and therefore the perpendicular magnetic recording type is suitable for increasing the recording density.
As a magnetic recording medium for use in the perpendicular magnetic recording type, a CoCrPt—SiO2 perpendicular magnetic recording medium (refer to Non-Patent Document 1) has been proposed because of high thermal stability and excellent recording characteristic. This is to configure a granular structure in a magnetic recording layer in which a non-magnetic grain boundary part with segregation of SiO2 is formed between magnetic particles in which a crystal with an hcp structure (a hexagonal close-packed crystal lattice) of Co continuously grows in a columnar shape, thereby achieving finer magnetic particles and an improvement of a coercive force Hc together. It is known that an oxide is used for the non-magnetic grain boundary (a non-magnetic portion between magnetic particles), and, for example, using any one of SiO2, Cr2O3, TiO, TiO2, and Ta2O5 has been proposed (Patent Document 1).
The magnetostatic characteristic and electromagnetic conversion characteristic of a magnetic layer having the granular structure can be adjusted by changing kinds of oxides that form the grain boundary or changing oxide contents. Both high coercive force and low noise are important, but they have a tradeoff relationship that, as one of them increases, the other decreases. Therefore, conventionally, the magnetic recording layer is divided into plural layers so that they take different roles. For example, by providing a layer that contains a smaller amount of oxide to achieve improvement in coercive force Hc and a layer that contains a larger amount of oxide to achieve improvement in SNR (Signal to Noise Ratio), the advantages of both the oxides can be obtained.
However, when an intense magnetic field is applied to the magnetic recording layer, leak field to an adjacent track becomes large, so that WATE (wide Area Track Erasure), namely, a phenomenon, that recorded information within the range of several micrometers from a track to write on is lost, is problematic. As means for reducing WATE, it is important to set a reversed magnetic domain nucleation field Hn of the magnetic recording layer at a negative value, and increase the absolute value thereof. In order to obtain a high (large in absolute value) Hn, a CGC (Coupled Granular Continuous) medium in which a thin film having a high perpendicular magnetic anisotropy is formed above or below the magnetic recording layer having a granular structure was devised (Patent Document 2).
The CGC medium shown in Patent Document 2 has a structure in which a CoB magnetic film and a Pd non-magnetic thin film are stacked one on top of another, and uses their exchange coupling to obtain a high Hn. However, in the CGC medium, the exchange coupling cannot be obtained unless the magnetic film is a thin film, and it is required to stack CoB films and Pd films alternately three times because only one for each layer is less effective. Therefore, in recent years, it is often found that an auxiliary recording layer which is a single layer having high perpendicular magnetic anisotropy and which is magnetically approximately continuous in an in-plane direction of a main surface of a base is formed on the magnetic recording layer.
As the coercive force Hc of the magnetic recording layer is more improved, a higher recording density can be achieved but writing by the magnetic head tends to be more difficult. Then, an auxiliary recording layer improves saturated magnetization Ms and also contributes to improving easy writing, namely, an overwrite characteristic. In other words, objects of disposing the auxiliary recording layer on the magnetic recording layer are to improve the reversed magnetic domain nucleation filed Hn to reduce noise, and to improve the saturated magnetization Ms to improve the overwrite characteristic. Note that the auxiliary recording layer may be referred to as a continuous layer or cap layer.